In one aspect the present invention relates to a novel spacer composition useful in downhole wellbore cementing operations. In another aspect, this invention relates to novel spacer compositions tailor weighted for use in downhole cementing applications. Still a further aspect of this invention relates to a method for cementing boreholes which includes the use of a spacer composition having excellent stability at temperatures at least as high as 500.degree. F., for example.
Conventional drilling procedures often include the placement of pipes or casings into the borehole to facilitate further drilling operations. The annulus between the outside wall of the casing and the inside wall of the borehole is filled with a mixture composed primarily of cement and water to thereby fix the casing rigidly in the borehole. Most conventional drilling operations also employ a drilling fluid, also known as drilling mud, used to carry the tailings out of the borehole and facilitate drilling. These drilling fluids will normally be present in substantial amounts of the wellbore as the casings are placed therein. Thus, a conventional method for placing cementing compositions is to pump them down the interior of the casing so that upon reaching the bottom of the casing they pass upwardly into the annulus between the casing and the borehole, displacing the mud and filling this annulus with cement. The cement, after hardening in the annulus, serves to fix the casing rigidly in the borehole.
The problem of obtaining a good cementing job is one of displacing substantially all of the drilling fluid in the annulus with the cement composition. Unfortunately, many of the drilling fluids employed are not compatible with oil well cements so that when the cementing composition comes into contact with the drilling fluids the mixture of cement and drilling fluid may flocculate or gel to form a material of complex chemical composition which is extremely viscous such that it becomes substantially unpumpable with conventional pumps and thereby prevents pumping of all the cement out of the interior of the casing and into the annulus. This can result in plugging of the interior of the well casing or, at the very least, channeling of the cement in the annulus so that the cement does not form a continuous phase completely around the annulus. Also, this gel can plug the annulus causing a buildup of pressure by the cement composition behind it to a degree sufficient to fracture the formation and pump cement into the formation. This can even plug the oil and gas producing formation of the well. With cement channels comes incomplete, and therefore weakened, cementing jobs which can allow fluids to leak away from the borehole being drilled and be lost in the incompletely cemented annulus and/or any porous formation adjacent thereto. Also, unwanted water may enter the casing from the formation in the same manner. The ultimate consequence is substantial economic and drilling efficiency loss.
In order to alleviate the problems which occur when well cement is allowed to contact or otherwise admix with drilling muds, of either the water base or oil base variety, spacer compositions have been used. Basically, the function of the spacer composition is to insulate, and act as a barrier between, the drilling mud present in the borehole and the cement composition. Thus, spacers are pumped into the borehole prior to introduction of the cement composition but after the drilling muds being employed. The requirements for spacer compositions include compatibility with both drilling muds and well cement compositions. Further, it is important that the spacer compositions possess sufficient stability such that the physical characteristics thereof do not change drastically at elevated temperatures which are sometimes present in boreholes. An example of a spacer composition is set forth in U.S. Pat. No. 3,799,874 to Parker. That composition is disclosed to be especially useful with oil base drilling fluids and comprises a water-in-oil emulsion in combination with an additive effective to prevent gelation of the spacer upon contact with a cementing composition.
Because both oil based drilling fluids and water based drilling fluids are employed in the field, a spacer composition which is compatible with either type of drilling fluid would be especially desirable. Further, a spacer composition which retains its stability at relative high borehole temperatures, of at least about 500.degree. F. would be especially desirable.